The injection of hot water into the top layer of soil for controlling nematodes and other pathogenic organisms is disclosed in U.S. Pat. No. 5,259,327. It has been found that under certain conditions, the treated top layer of soil tends to cool rapidly due to exposure to the surrounding air. Needless to say, the cooling effect is more rapid during periods when the air temperature is cool and/or when it is raining. If the soil cools down too quickly after treatment, the desired effect, control of pathogenic organisms and especially weed seeds, may not be achieved.